1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation of the type having an x-ray source with exposure parameters which are set in a parameter setting unit, and an x-ray image intensifier video chain which includes a processing circuit connected to a video pick-up unit and which includes a motion detector, the processing circuit controlling the parameters of a filter unit for the chronological or temporal filtering of the video signal.
2. Description of the Prior Art
X-ray diagnostics installations of the type generally described above effect a reduction in the quantum noise, and temporal noise such as, for example, electrical noise in the x-ray image.
Exposures are made in x-ray diagnostics installations with a radiation dose which is as low as possible in order to protect the patient from excessive radiation exposure. As a result, however, quantum noise constitutes an unavoidable physical limit in such systems. In order to reduce the quantum noise, chronological low-pass filtering or chronological averaging of the video signals are used in the production of image sequences, as disclosed, for example, in German OS 34 26 830. The present of chronological changes within the video signal, i.e., differences in the subtraction image, are thereby detected by means of a threshold circuit. Parameters of the filter circuit, for example the time constant, are then modified pixel-by-pixel based on the output of the threshold circuit, so that moving image portions are reproduced with a slight low-pass effect but noisy, stationary image parts are reproduced with a high degree of low-pass filtering, and are thus displayed with a reduced noise content.
A compromise among the radiation dose, the presentations of motion in the displayed image, and the chronological filtering must be made in order to obtain an optimum image quality even for chronologically changing events such as, for example, fast movements.
A problem in such known systems, however, is that making a distinction in subtraction image between frame-to-frame differences caused by noise and the frame-to-frame differences caused by motion is extremely difficult. Chronological filtering which is different for the individual pixels can thus not be controlled in an accurate manner.
German OS 31 21 597 discloses a system for reducing noise in video signals, including a motion detector which weights the current video signal and a delayed video signal. For producing a signal indicative of motion, the motion detector includes a subtraction unit followed by circuitry which is controlled by a control voltage derived from the video signal, and which represents the amplitude of the noise.
An automatic image quality compensation method and apparatus are disclosed in German OS 42 40 215 which employ fuzzy logic for improving the image quality and, among other things, improving the signal-to-noise ratio. For this purpose, the brightness and the sharpness of the image content, as well as the signal-to-noise ratio in the video signal are analyzed using fuzzy logic and are compensated dependent on this analysis.